This invention relates to rotary drill bits used to drill oil and gas wells in the earth, and more particularly to rotary drill bits of the so-called sealed bearing roller cutter type.
This invention involves an improvement on the prior art sealed bearing roller cutter drill bit, such as shown for example in U.S. Pat. Nos. 3,397,928 and 3,765,495, comprising a bit body having a threaded pin at its upper end adapted to be detachably secured to a drill string for rotating the drill bit, and three depending legs at its lower end each having a generally cylindrical bearing journal. Conical roller cutters having a recess of generally circular section in the end thereof are rotatably mounted on the bearing journal by bearing means. A lubrication system provides lubricant to each bearing means. The system comprises a reservoir in the bit body holding a supply of lubricant, passaging in the bit body providing fluid communication between the reservoir and the bearing means for flow of lubricant to the bearing means, and a seal member between the roller cutter and the bearing journal for blocking flow of lubricant past the bearing means and out of the bit body. The seal member is a ring of elastomer material carried in an annular groove or recess in the roller cutter and is radially compressed against the bearing journal.
Because of the limited quantity of lubricant in the lubrication system, the seal member must be effective in blocking egress of lubricant from the bit body so as to prevent the bearing from running "dry" and thus destruction of bearing. Similarly, because the drilling fluid surrounding the bit when in a well bore is highly abrasive, the seal member must also be effective in blocking ingress of drilling fluid into the bit body. To make the seal substantially leak-proof, the seal is typically subject to a high degree of radial compression (i.e., 10% or more compression as disclosed in U.S. Pat. No. 3,397,928, and something approaching but not reaching 10% compression as disclosed in U.S. Pat. No. 3,765,495). Such levels of compression are significantly higher than that recommended for other uses of elastomeric sealing rings. Moreover, it is the accepted practice in the industry, to make the face of the seal member which is in sliding, frictional engagement with the bearing journal a smooth, flawless surface.
In this latter regard, each of the competitors in the drill bit industry has developed quality control procedures to ensure that the engaging face of each drill bit seal member is free of surface imperfections. These quality control procedures are based on the "Rubber Manufacturers Association O-Ring Inspection Guide Handbook" and Military Standard (MIL-STD-413B) for Visual Inspection Guide for Elastomeric O-Rings. The inspection procedure of the assignee of this application, for example, requires that the engaging face of the seal member be a smooth, uniform, continuous surface free of all of the fourteen kinds of unacceptable surface imperfections described in MIL-STD-413C, when the seal member is visually examined under 3.times. magnification. It has been the accepted wisdom in the industry that surface imperfections are highly undesirable in that an imperfection may provide a flow path (or at least a portion of a flow path) for lubricant or drilling fluid leakage. In addition, such surface imperfection could also cause stress concentrations in the highly compressed seal member which may result in tearing of the the seal member due to the high frictional forces on the engagement face of the seal member tending to shear the face from the remainder of the seal member.
Seal members having smooth engagement faces have been generally satisfactory for sealed bearing roller cutter drill bits as evidenced by the fact that such seal members have been the industry standard since the early 1970's. However, in recent years, improvements in drilling equipment technology has enabled drillers to turn drill bits faster so as to deliver more power to the drill bit for higher rates of drilling penetration through the earth's formations, and a problem, described more fully hereinafter, has developed with this seal member.
A modern rotary drill rig can rotate the drill string and thus a roller cutter drill bit at 150 rpm as compared to an older drill rig which could rotate at only 110 rpm. In addition, positive displacement down hole motors with speeds of rotation of up to 500 rmp and down hole turbine motors with speeds of up to 1000 rmp, which had previously been used only with diamond drag drill bits, are not being used to rotate roller cutter type drill bits. Because each roller cutter rotates approximately 11/2 times for each rotation of the drill bit body and the length of the circumference of the bearing journal engaged by the seal member increases with drill bit body diameter, seal members (particularly those in large diameter bits) are now subject to high linear speeds, as measured in feet per second, at their engagement faces. For example, in one common application in which a 171/2 inch diameter drill bit is run on a turbine motor at 750 rpm, the engagement face of the seal member is subject to a linear speed in excess of 20 feet per second. Such speeds exceed the recommended speed for an O-ring seal member used in even the most ideal conditions, much less in a drill bit in which the seal member is highly compressed, is exposed to highly abrasive material (i.e., the drilling fluid) and to high operating temperatures, and is carried in a bearing member (i.e., the roller cutter) that wobbles and reciprocates, as well as rotates on the bearing journal. With these high speeds of drill bit rotation, the smooth surface seal members have shown a tendency to fail well before the remainder of the drill bit is fully worn.
The reasons for such failure are not fully understood. However, applicants believe that this is due in significant part to an inability of the seal member to adequately hold lubricant to its engagement face, with the result that the engagement face becomes overheated. Excess heat in the seal member causes vulcanization of the seal member's elastomeric material, thereby resulting in the seal member losing its elasticity and hence its ability to remain in sealing engagement with the bearing journal as the roller cutter wobbles. Excess heat thus results in leakage of lubricant and/or drilling fluid past the seal member, and shortly thereafter failure of the bearing and the entire drill bit.